5 Suggested Answers

Hi,
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These models are known for this issue.
I've had my share in the shop here.
Disconnect all speakers,turn speakers off, try headphones only
If it still persists then there is either :
1. power supply capacitors gone bad or
2. The thermal fuse in the transformer is open.
A tech can very easily find out if the thermal fuse is open by checking the primary of the transformer
if the plastic is shrinking away from the power supply capacitors then they've gone bad

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Since you have 2 receivers, you can either use the Sony STR-AV570X or the TA-AX380. You will need 3 Stereo RCA Cables to connect your CD Player, Tuner, and Tape Player. The speakers connect to the receiver you choose using the speaker input terminals.

I can see why you're asking for help on this. It almost seem this unit has been 'disappeared' as far as online information is concerned. Even Sony has nothing on it.

I founs some articles and For Sale posts:

http://toronto.kijiji.ca/c-buy-and-sell-electronics-Sony-STR-4800SD-Stereo-Receiver-135-OBO-W0QQAdIdZ243602516
"Sony's STR-4800SD receiver was extensively, and positively, reviewed in Stereo magazine's Fall '77 issue. In their introduction, they found the 4800SD bettered its published f.m. sensitivity specs "by a wide margin...it has exceptionally smooth frequency response, very low distortion(especially in the midband), and excellent quieting. No exotic claims are made for the power amp section, either (35 watts/channel,15.4 dBw at 0.2% THD). But our tests indicate much more capability than that, and a distortion of only one-tenth that spec."

The power consumption figure shown in a photo below says "145W", which would be consistent with a 35 watt amplifier.

This can be shorted output transistors (common) or a defective voltage amplifier IC. WIthout taking measurements inside, there is no way for me to make that determination. In any case, this should be a relatively simple repair for a shop that repairs audio gear. Expect a parts cost in the $25-$40 range.